Arousal from hibernation increases blood oxygen saturation in 13-lined ground squirrels.

Hibernating Ictidomys tridecemlineatus, 13-lined ground squirrels, are considered models of ischaemia-reperfusion tolerance, as both tissues and isolated mitochondria withstand anoxia followed by rapid re-oxygenation in vitro. This tolerance is likely adaptive, protecting against damage during the numerous arousals from torpor throughout the hibernation season. O2 availability is likely low during torpor, but suppressed metabolism lowers O2 demand, potentially mitigating hypoxic stress.

Gas exchange, oxygen transport and metabolism in high-altitude waterfowl.

High-altitude life poses physiological challenges to all animals due to decreased environmental oxygen (O) availability (hypoxia) and cold. Supporting high metabolic rates and body temperatures with limited O is challenging. Many birds, however, thrive at high altitudes.

The catecholamine response to graded high-altitude flight in yellow-rumped warblers ().

Chronic exposure to low oxygen (hypoxia) leads to amplification of the hypoxic chemoreflex, increasing breathing and sympathetic nervous system (SNS) activation. Prolonged SNS activation redistributes blood to hypoxia-sensitive tissues away from muscles. Recent tracking studies have shown that migratory songbirds can fly 5,000 m or higher above sea level, leading us to hypothesize that migratory birds may have a blunted hypoxic chemoreflex to maintain blood flow to muscles during migratory flight at high altitudes.

A comparison of seasonal flexibility in pectoralis muscle fiber type and enzyme activity in migratory and resident sparrow species.

The pectoralis muscle in birds is important for flight and thermogenesis. In migratory songbirds this muscle exhibits seasonal flexibility in size, but whether this flexibility reflects changes in muscle fiber type has not been well documented. We investigated how seasonal changes in photoperiod affected pectoralis muscle fiber type and metabolic enzymes, comparing among three closely related sparrow species: two seasonal migrants and one year-round, temperate climate resident.

On the Physiology of High-altitude Flight and Altitudinal Migration in Birds.

Many bird species fly at high altitudes for short periods and/or shift seasonally in altitude during migration, but little is known about the physiology of these behaviors. Transient high-altitude flight, or short-term flight at extreme altitudes, is a strategy used by lowland-native birds, often in the absence of topographic barriers. Altitudinal migration, or seasonal roundtrip movement in altitude between the breeding and non-breeding seasons, is a form of migration that occurs as a regular part of the annual cycle and results in periods of seasonal residency at high altitudes.

Adaptation in brain structure and respiratory and olfactory structures across environmental gradients in African and North American muroid rodents.

Morphometric studies of 3D micro CT-scanned images can provide insights into the evolution of the brain and sensory structures but such data are still scarce for the most diverse mammalian order of rodents. From reviewed and new data, we tested for convergence to extreme aridity and high elevation in the sensory and brain morphology of rodents, from morphometric data from micro-CT X-ray scans of 174 crania of 16 species of three distantly related African murid (soft-furred mice, Praomyini, laminate-toothed rats, Otomyini, and gerbils, Gerbillinae) clades and one North American cricetid (deer mice and white-footed mice, Peromyscus) clade. Recent studies demonstrated convergent evolution acting on the oval window area of the cochlea (enlarged in extremely arid-adapted species of Otomyini and Gerbillinae) and on endocranial volume (reduced in high elevation taxa of Otomyini and Peromyscus).

Migratory songbirds exhibit seasonal modulation of the oxygen cascade.

Migratory flight requires birds to maintain intensive aerobic exercise for many hours or days. Maintaining O2 supply to flight muscles is therefore important during migration, especially since migratory songbirds have been documented flying at altitudes greater than 5000 m above sea level, where O2 is limited. Whether songbirds exhibit seasonal plasticity of the O2 cascade to maintain O2 uptake and transport during migratory flight is not well understood.

The relationship between hypoxia exposure and circulating cortisol levels in social and solitary African mole-rats: An initial report.

Hypoxemia from exposure to intermittent and/or acute environmental hypoxia (lower oxygen concentration) is a severe stressor for many animal species. The response to hypoxia of the hypothalamic-pituitary-adrenal axis (HPA-axis), which culminates in the release of glucocorticoids, has been well-studied in hypoxia-intolerant surface-dwelling mammals. Several group-living (social) subterranean species, including most African mole-rats, are hypoxia-tolerant, likely due to regular exposure to intermittent hypoxia in their underground burrows.

Gene regulatory changes underlie developmental plasticity in respiration and aerobic performance in highland deer mice.

Phenotypic plasticity can play an important role in the ability of animals to tolerate environmental stress, but the nature and magnitude of plastic responses are often specific to the developmental timing of exposure. Here, we examine changes in gene expression in the diaphragm of highland deer mice (Peromyscus maniculatus) in response to hypoxia exposure at different stages of development. In highland deer mice, developmental plasticity in diaphragm function may mediate changes in several respiratory traits that influence aerobic metabolism and performance under hypoxia.

Counter-gradient variation and the expensive tissue hypothesis explain parallel brain size reductions at high elevation in cricetid and murid rodents.

To better understand functional morphological adaptations to high elevation (> 3000 m above sea level) life in both North American and African mountain-associated rodents, we used microCT scanning to acquire 3D images and a 3D morphometric approach to calculate endocranial volumes and skull lengths. This was done on 113 crania of low-elevation and high-elevation populations in species of North American cricetid mice (two Peromyscus species, n = 53), and African murid rodents of two tribes, Otomyini (five species, n = 49) and Praomyini (four species, n = 11). We tested two distinct hypotheses for how endocranial volume might vary in high-elevation populations: the expensive tissue hypothesis, which predicts that brain and endocranial volumes will be reduced to lessen the costs of growing and maintaining a large brain; and the brain-swelling hypothesis, which predicts that endocranial volumes will be increased either as a direct phenotypic effect or as an adaptation to accommodate brain swelling and thus minimize pathological symptoms of altitude sickness.

Genetic variation in HIF-2α attenuates ventilatory sensitivity and carotid body growth in chronic hypoxia in high-altitude deer mice.

The gene encoding HIF-2α, Epas1, has experienced a history of natural selection in many high-altitude taxa, but the functional role of mutations in this gene is still poorly understood. We investigated the influence of the high-altitude variant of Epas1 in North American deer mice (Peromyscus maniculatus) on the control of breathing and carotid body growth during chronic hypoxia. We created hybrids between high- and low-altitude populations of deer mice to disrupt linkages between genetic loci so that the physiological effects of Epas1 alleles (Epas1 and Epas1 , respectively) could be examined on an admixed genomic background.

Physiological and behavioural strategies of aquatic animals living in fluctuating environments.

Shallow or near-shore environments, such as ponds, estuaries and intertidal zones, are among the most physiologically challenging of all aquatic settings. Animals inhabiting these environments experience conditions that fluctuate markedly over relatively short temporal and spatial scales. Living in these habitats requires the ability to tolerate the physiological disturbances incurred by these environmental fluctuations.

Adrenergic control of the cardiovascular system in deer mice native to high altitude.

Studies of animals native to high altitude can provide valuable insight into physiological mechanisms and evolution of performance in challenging environments. We investigated how mechanisms controlling cardiovascular function may have evolved in deer mice () native to high altitude. High-altitude deer mice and low-altitude white-footed mice () were bred in captivity at sea level, and first-generation lab progeny were raised to adulthood and acclimated to normoxia or hypoxia.

Genetic variation in haemoglobin is associated with evolved changes in breathing in high-altitude deer mice.

Physiological systems often have emergent properties but the effects of genetic variation on physiology are often unknown, which presents a major challenge to understanding the mechanisms of phenotypic evolution. We investigated whether genetic variants in haemoglobin (Hb) that contribute to high-altitude adaptation in deer mice (Peromyscus maniculatus) are associated with evolved changes in the control of breathing. We created F2 inter-population hybrids of highland and lowland deer mice to test for phenotypic associations of α- and β-globin variants on a mixed genetic background.

Distinct Mechanisms Underlie Developmental Plasticity and Adult Acclimation of Thermogenic Capacity in High-Altitude Deer Mice.

Developmental plasticity can elicit phenotypic adjustments that help organisms cope with environmental change, but the relationship between developmental plasticity and plasticity in adult life (e.g., acclimation) remains unresolved.

The adaptive benefit of evolved increases in hemoglobin-O affinity is contingent on tissue O diffusing capacity in high-altitude deer mice.

Background: Complex organismal traits are often the result of multiple interacting genes and sub-organismal phenotypes, but how these interactions shape the evolutionary trajectories of adaptive traits is poorly understood. We examined how functional interactions between cardiorespiratory traits contribute to adaptive increases in the capacity for aerobic thermogenesis (maximal O consumption, V̇Omax, during acute cold exposure) in high-altitude deer mice (Peromyscus maniculatus). We crossed highland and lowland deer mice to produce F inter-population hybrids, which expressed genetically based variation in hemoglobin (Hb) O affinity on a mixed genetic background.

Evolution and developmental plasticity of lung structure in high-altitude deer mice.

Hypoxia at high altitudes can constrain the ability of endotherms to maintain sufficient rates of pulmonary O transport to support exercise and thermogenesis. Hypoxia can also impede lung development during early post-natal life in some mammals, and could thus accentuate constraints on O transport at high altitude. We examined how these challenges are overcome in deer mice (Peromyscus maniculatus) native to high altitude.

Life-long exposure to hypoxia affects metabolism and respiratory physiology across life stages in high-altitude deer mice ().

Hypoxia exposure can have distinct physiological effects between early developmental and adult life stages, but it is unclear how the effects of hypoxia may progress during continuous exposure throughout life. We examined this issue in deer mice () from a population native to high altitude. Mice were bred in captivity in one of three treatment groups: normoxia (controls), life-long hypoxia (∼12 kPa O from conception to adulthood) and parental hypoxia (normoxia from conception to adulthood, but parents previously exposed to hypoxia).

Coordinated changes across the O transport pathway underlie adaptive increases in thermogenic capacity in high-altitude deer mice.

Animals native to the hypoxic and cold environment at high altitude provide an excellent opportunity to elucidate the integrative mechanisms underlying the adaptive evolution and plasticity of complex traits. The capacity for aerobic thermogenesis can be a critical determinant of survival for small mammals at high altitude, but the physiological mechanisms underlying the evolution of this performance trait remain unresolved. We examined this issue by comparing high-altitude deer mice () with low-altitude deer mice and white-footed mice ().

Ontogenesis of evolved changes in respiratory physiology in deer mice native to high altitude.

High-altitude environments are cold and hypoxic, and many high-altitude natives have evolved changes in respiratory physiology that improve O uptake in hypoxia as adults. Altricial mammals undergo a dramatic metabolic transition from ectothermy to endothermy in early post-natal life, which may influence the ontogenetic development of respiratory traits at high altitude. We examined the developmental changes in respiratory and haematological traits in deer mice () native to high altitude, comparing the respiratory responses to progressive hypoxia between highland and lowland deer mice.

Cardiovascular responses to progressive hypoxia in ducks native to high altitude in the Andes.

The cardiovascular system is critical for delivering O to tissues. Here, we examined the cardiovascular responses to progressive hypoxia in four high-altitude Andean duck species compared with four related low-altitude populations in North America, tested at their native altitude. Ducks were exposed to stepwise decreases in inspired partial pressure of O while we monitored heart rate, O consumption rate, blood O saturation, haematocrit (Hct) and blood haemoglobin (Hb) concentration.

Cardiovascular responses to progressive hypoxia in ducks native to high altitude in the Andes.

The cardiovascular system is critical for delivering O2 to tissues. Here we examine the cardiovascular responses to progressive hypoxia in four high-altitude Andean duck species compared to four related low-altitude populations in North America, tested at their native altitude. Ducks were exposed to stepwise decreases in inspired partial pressure of O2 while we monitored heart rate, O2 consumption rate, blood O2 saturation, haematocrit (Hct), and blood haemoglobin concentration [Hb].